A writefreely contact form
Thanks to dejalavidavolar and a simple Javascript injection into the custom CSS, it's possible to use a Letterbird contact form in writefreely.
You simply embed a simple script into the Customise ... CSS window, using the relevant Letterbird username and the URL of the page it will be on. Don't forget to add the terminating <style type="text/css"> so that the CSS is correctly incorporated.
Edit your writefreely contact page to include the following text:
<div id="letterbird-container"></div>
Remember, the URL within the script and the URL of the page accessed containing the form must be a perfect match ... this means that a draft version of the post (which has a random filename) will not display the form.
I've not managed to find a way to display the graphics or header text that Letterbird forms can also incorporate (but, then again, I've not tried very hard either).